1. Field of the Invention
The present invention is related with the measurement of mechanical properties of the pulmonary system of a patient and more particularly with the determination of various clinically significant compliance and related parameters of the lung-chest system.
The study of lung mechanics implies the determination of the elastic properties of the lung-chest system. The elastic properties are quantified by successive inflation and deflation of the patient's lungs thereby inducing predetermined air or gas volume changes while simultaneously measuring the corresponding pressure changes.
From the volume/pressure (V/P) relationship, the following parameters (=typical or clinically significant volume to pressure ratios) can be derived:
(a) Total compliance (V/p), wherein V is the inflation volume, of usually 10-15 ml per kg patient weight.
(b) Starting compliance, i.e. the slope of the V/P-curve in its initial portion.
(c) Inflation compliance, i.e. the slope of the V/P curve in its second steeper portion during inflation.
(d) Inflection point or opening pressure, i.e. the knee of the V/P curve between the initial and the steeper portion.
(e) Deflation compliance, i.e. the slope of the V/P curve during deflation.
(f) Unrecovered or "trapped" volume, i.e. the volume in the pulmonary system at the end of deflation, at zero pressure.
(g) Hysteresis area, i.e. the area surrounded by the V/P-curve during inflation and deflation.
(h) Hysteresis ratio, i.e. the hysteresis area divided by the area V.sub.max.P.sub.max.
(i) Best PEEP pressure, i.e. the pressure at which the inflation compliance portion of the V/P curve becomes essentially linear.
2. Background Art
The pressure-volume curve in paralysed patients is presently obtained in two ways: Firstly, by manual inflation and deflation by means of a so-called super-syringe with simultaneous reading of pressures in respiratory ways or, secondly by feeding a constant air or gas flow from a compressed-gas flow generator and independent measurements of resulting pressures.
For obtaining the above-mentioned parameters (a) to (i), complex calculations are required, to be performed separately, which require a long time. Furthermore, the results were not precise enough to be really clinically significant.